Control system for high-frequency induction heating apparatus



H. F. STORM 2,521,886 CONTROL SYSTEM FOR HIGH-FREQUENCY INDUCTIONHEATING APPARATUS Sept. 12, 1950.

2 Sheets-Sheet 2% Filed Sept. 25, 1946 Time in Seconds Inventor Herbert F Storm By y HNHNNNO Sept. 12, 1950 H. F. STORM 2,521,880

- con'mor. svs'rsm FOR HIGH-FREQUENCY Y mnqc'rzou mmme APPARATUS Filed Sept. 25, 1946 2 Sheets-Sheet 2 Patented Sept. 12,4950

f CONTROL SYSTEMFOR nien rnsotiimor INDUCTION'HEATINGnBlPARATUS" Herbert F. Storm,- Scotia, N. assignorto Sun- -beam Corporation, Chicago,

of Illinois 111., a corporation Application September 25, 1946, s iniN sea19e and electrical resistivity of the work piece or charge. Mathematically, the rate of heat generation or the heat generated per unit of time in a high frequency induction heating device can be represented. by the following equation:

Faith/77; Where P isthe heating rate in B. t. u.s per second; K is a constant;- ills the current in amperes flowing in the inductor =.0r, heating coil; f is the frequency of the current i; is the permeability in webers per ampere-turn per centimeter of the work piece or charge to beheated; and p is the resistivity of the work piece in ohms per cubic centimeter. In View of the fact that most materials have a positive temperature coefi'icient of electrical resistivity,-it is apparent from the equation that ,the heat generated in the charge will vary with temperature. As the temperature increases during a heating cycle, the electrical resistivity for a material having a positive temperature coefficient also increases andhence the rate of heat generation by induction increases. High frequency generators which are used in induction heating devices and which generally comprise oscillation generators should be utilized to the maximum extent during a heating cycle in order to utilize the invested capital, and to increase the efliciency as willbe explained hereinafter. Accordingly; it is desirable to attempt to load the generator as high as is permissible throughout the heating cycle. It is, however, important that the load on the oscillation generator does not exceed a predetermined maximum. If one were to plot the rate of heat generation in a charge as a function of timejthe curve would start at zero andincrease with time due to the increasing electrical re sistivity of the charge with temperature, assuming that allother variables remained constant. It is therefore apparent that in such a case during most of the heating cycle the oscillation generator is operated far below its permissible maximum which is attained only as the end temperature of the workpieceis reached. Itwould bedesirable to increase the-utilization'of the gen- 1. .1 8 Claims. (c1. glean) erator; thereby decreasing the heating time for each particular charge or work piece with the resultant increase in production and efficiency of r the induction heating Accordingly,-it is an object of the present invention to increase the utilization of agenerator supplying high frequency energy for induction heatingvpurposes so thatthe generator is operated more efliciently throughout heating cycle. 4

ltis another object of the present invention to provide'an induction heating unit in which the-generator supplying the high frequency energy reaches its maximum output before the work piece reaches its desired end temperature. 1 It is another object of the present invention to provide an automatic control arrangement fora-high frequency heating device in which the load on the high frequency generator is pre-' ventedfrom exceeding a predetermined value and yet wherein the eiliciency of the generator is increasedabove that of prior art devices.

. It'is 'a further object of the present invention toprovide an automatic control arrangement for a higli frequency heating unit employing a high frequency generator in order to tend to maintain constant the power output of the high frequency. generator throughout the heating cycle.

Further objects and advantages of the present invention will become apparent as the following description proceeds and the features of novelty which characterize this invention will be pointed outkwith particularity in the claims annexed to and forminga part of this specification.

-'Fori:a better understanding of the presentinvention reference may be had to the accom panying? drawings in which i'Figs. land-2 are curve diagramsto aid in understanding. the present invention 3 is a schematic diagram illustrating a high frequency. heating device embodying 'the present invention;

F Figs. 4 and: 5 are vector diagrams to aid in understanding the operation of the heating de- Fig. 6 is a schematic diagram of a portion of- The present invention is primarily concerned with an automatic control arrangement for-a high frequency heating device in which the rate of heating is controlled so that maximum power is trans'mitted before'the charge or work piece has reachedits desired end temperature, wherethe entirethe desired enditemperature is reached. This is:

based primarily upon the fact that the electrical resistivity of the charge or work piece increases.

with temperature so that .the heating of the charge increases as the worldpiece becomes-"not This is true for all materials having a posi-' ter. tive temperature coefficient ofelectrical resistiv ity except ferromagnetic materials and is even- In my copending application, 'Serial No1'69'9,1'97,"

.1 less than the time T1.

filed concurrently with the present application and assigned to the same assignee as the present application, there is disclosed-and claimed an are.

rangement'for controlling abish frequency heating device which specifically takes. into. c nsiderati n th han n the rate-oi-heetinawith the reat change in, permea ty o casi ned when a ferromagnetic charge or work piecemeacheS :its recalescent point.

Referring now to Fig. 1 of thdrawingsthere.

is illustrated a curvedwhich shows therate 01' heat generation P as a function oftime. This curve is plotted forthe ,conventional induction heating ice in which th rate oiheating-increases until the time T1. when the desired-end temperature of t e work piece is reached and which represents themaximummfirmissible rate of heating Pmax. It is :apparentthat during most of the heating ycle and certainly during the first part of the heating cycle the generator su plying the power is not operatedivery efficiently.-

n accordance with the present invention, in order to produce a .hightntilizationnof the high fr q y generator; he inductor orheating coil is designed to transmit. maximum .power at a temperature belowthe desired end temperature of the work piece. .As soon-as the temperature at which maximum power .is transmitted isreached, the inductor current must be reduced in order to prevent overloading of the generator due to further increase of the load resistance with-increase in temperature by virtue .of increasedelectrical resistivity. This can be more .clearlywunderstood by reference toFig. '2 ofthe drawings in whichthecurve aof Fig.= 1 is redrawn to a'slighlw ly different scale,--.thereby enabling theiprinciple propounded hereinafter to. be explained :in greater detail. As time increases, .the rate .of .heat'tgeneration also increasesz'and it reacheswits p eak Pmax at A in the time-1T1 when-the charge .or work piece attains its final end temperature. If now the design of the output circuit were changed in order to increase the.rate'ofheat-generation by. a predetermined amount, the heat Jgenerator'for this. changed arrangement can be represented by the curve 2) in Fig. 2 .of the drawings." Duezto the greater power input, the temperature of the charge rises faster than was the .case in connection with curve (1.1'1119 maximum permissible. loading of the generator reachedratathepoint B. It is obvious that if the heating. were continued without any readjustment of :the genera.- tor, the latter would become overloaded. In order toavoid this condition, means are provided, :as will become apparent from the :following description, for reducing the output of thegenerator. The rate of heat generation will be reduced :as

shown by the curve b between the points B and B. During this time the heating rate is de:

creased below the maxirnumheatingn te b t it.

isistill greaterthan the heating losses duringthis period so that thetemperature of thecharge in curves in Fig. 2 of the drawings indicates that the end temperature of curve a is reached in the time 1 'Trwhile the'en'd temperature in the case of curve his reached'in the time T2 which is considerably It is apparent then that the utihzation of the generator has been greatly increased sinoe the same work has been accomplished in a shorter period of time and yet the permissible maximum rate of heating Plan has not been exceeded.

If the design of the output circuit were changed still .iurther so to transmit: sti11..m0repow during the earlier 'stases v oizths heatinatyele. ahc r-vcrsueh as curve c results tinhich the maxim m rate oi heatin occurs at point C and heating at a reduced ateoccurs betweenwthe points C and C. The desired end itflmperatmc is reached in the time It which is still less than the time T2 thus indicating that an even greater utilization or-the high frequency seneratoris obtained. If the design oi the output circuit is changed againhso-zthat maximum loading is reached at the 'uoint D. as represented by the curve d, then. between D and D the latent heat ing is reduced.- Since a comparatively large amount or power must be transmitted after the maximum rate of heating at point!) is reached and :this heating occurs at a lower outputlevel, a greater amount ci time will be required to bring the charge to the desiredzendatemperature.

3% It should be noted at this point that the heat loss per unit time increases with the temperature of-the work piece byi -virtue :oi the -higher temperature differential, thug 1 decreasing the ex cess of the transmitted heat over the heat loss.

The-result that the time -that heat Imust'be supplied at a reduced rate becomes progressively longer as the maximumirate 0f heatingis reached earlier and-. earlier durin'g the cycle. In-the case of curve d no tin'iewasgainedrelative to curve 0' 1 since both required:"-the tinie Ta' to rea'ch the same end temp'erature ofthe-workpiece; *If the r at power transfer is stepped up'stili further so that the maximum rate: of heating- Pmsx 'is reached 'earlierwin the cyclone ior example in the manner-indicated by -the curve 6, it will be apparent that a point is reached a t which-the heat gerrerated during the reduced' heati-ng period-"after the point -E 'on curve' cis reached "when the heat generated in thechargeequails theheat losslrom the charge. If thiscondition-exists before the desired end tempera-tur'e'is attained, it-is appar ent that it will never be attained; Consequently. an indiscriminate increase in thereto or "heat generation early in 'theheating' cycle will not necessarily reduce theitotal time required for tliel e g le which he final analy sis the peration is completed-and thedesired' endtem their? is re he n. the lmmim mr r ief: time- H .sR e emwto F .101) rawinea h r u t' iet a gh fre enc eat neamnee: es t whic P rmi Pf; cqnt q li sqs h i ate of heat 'generation sothat the high, frequency genorator may be operated in. accordance with. a

curve such as c or id of llfig. 2 f the drawings.

in preference to curves such as a orb; It should be'understood that any suitable high frequency generator may be employed. Sinceit is customary for such high frequency'generators,to.be of thehighvacuum tube oscillator type the present invention hasv been particularlyillustrated. in connection with such a generator 'I Fig. 3 of the drawingsthere isiillustrateda control circuit for a high frequency induction heating device comprising aninductor or heats.

ing' coil [0. which usually comprisesa one turn water-cooled coil of the type disclosedin-my copending application, Serial No; 652,756,..filed March '7, 1946, and assigned to the same assignee as the present application.

'll'or the purpose of energizing highir'equency heating 0011 ID with high irequency current so that thework piece or charge, iotshown, associated. therewith isheated in an efficient and high speed manner, .there -is.pro-

vided an oscillation generator generally indicated. at I It should be understood that any suitable.

means for providing high frequency energy to. the

inductor or heating coil l0 may beemployed and accordingly, the oscillation generator. Il .may

comprise any standard form of generator for pro-* ducing high frequency oscillations. In Fig. 3 of the drawings this generator has been illustrated as comprising the well known Colpitts oscillator including an electron discharge valve-or vacuum tube l2 having an anodeor plate 13, a cathode 14 and I 3-B0llfiYQliB1BClJTOdB or. grid 1 I 5. The. oscill4,.of the electron discharge valve -12 is con nected to a point on the tank circuit/between the capacitors IT and I8 which act as a voltage divider. The lower terminal of the tank circuit is connected tothe control electrode or grid l through a grid blocking capacitor l9. This connection from the tank circuit to the control electrode provides the feed-back circuit which is a capacitive feed-back circuit. The upper terminal of the tank circuit is connected to the plate oranode l3 through a. plate blocking condenser in order to insulate the anode or plate [3 from thecontrol electrode or grid IS in so far as the direct current potential appliedtothe anodecathode circuit of the electron discharge valve I2 is concerned while still maintaining the plate l3 andthe upper terminal of the tankcircuit atthe the inductorv orv Preferably the'cathode I4 is grclun ieizlas 6. c It will be understood that the oscillation generator-1; of1; the. type; described thus'far effectively converts giirect current energy; tohigh :frequency alternating current energy. Consequently, it is necessary to suppl the. iplate=to-c athode circuit of electron discharge valve. l2..which will be referred .tohereinafter as the.- input circuit of the oscillation generator-ll with asource of direct current potential. .zAny suitable source :of direct current potential may bev employed.- In order to perform the desiredcontrol operationstobedescribed hereinafter, however, there isprovided a source :of. alternating current potential- 25 whichis connected to the input circuit of the-oscillationgeneratorll througha control=device such as: -:a circuit breaker-or contactor schematicallyindicated at 28, an. electron discharge device'control. unitgenerally indicatedat 2], and a bi-phase rectifier .unit generally indicatedat 28. The biphase rectifier.- unit '28 converts the; alternating current. or potential from the source 25 to a di-- rect current potential while i: the electron discharge control unit zl controls the magnitude of the directcurre'nt potential applied to them--- put terminals or the-oscillation generator ll As :illustrated in Fig. 3 ofthe drawings;the

rectifier unit 28 comprises a bi-phas'erectifier 'including a rectifier transformer 29;having apri mary winding 3ll anda secondary winding 32.

The I end terminals {of 1 the secondary winding 32 are each connected to an anode'33 associated with a pair ofelectrondischar-ge valves 34 and '35; re-

spectively, which are indicated as ordinary reoti Each of fier tubes such as conventional diodes.- I the electron discharge'valvesordiodes 34 'and; 35

are provided with} a cathode BEinterc nne'cte d" as indicated at' 37 to 'provideone terminal of the 1 output circuit of*the-rectifier"unit28. :The' other" terminal of 'the'f-o'utput circuitof the rectifier unit 28 is the'mid-tap 38 of the secondary w ndin s:

of the: rectifier transformer 29: 1:1 order 'to smooth the outputpf bi-phase rectifier 28 to eliminate-the ripple which would otherwise'be included, there is provided a suitable by p'assjcon' denser 39 connected across the output terminals I of the rectifier to by-pass the" ripple component of the output voltage anda suitablechoke coilitEl to impede the passage of alternatingcurrent.

filter circuit also prevents the high frequency oscillations from passing into the rectifier unit28."

The primary winding 30 of 'the' rectifier trans former 29 is connected as was mentioned above to "the" alternating current sourcethrough the control device 26 and the electron discharge orelectronic control unit I 21'.

is provided in order to controhthe' magnitude.

of ithe direct current potential applied to the; input of the oscillation generator' ll and com -.3 quently to-control'the magnitudeof the joutput of oscillation generator II which appears in the employing an ionizable mediumsuch asIa'gas'ior a vapor. Examples of these'valves are the thyratron orlthe ignitron. Each'of the electronfdis char'ge'valves 42 and 43 is provided with an anode 44, a cathode 45, and a control electrode 46.

These electron discharge valves areconnected' in a kr e rs a i0nsh inseries Withit p .75 niary winding an or the rectifleretransfcrmer. :29...

The 'ele'ctron" elicscharge control unit21, as was mentioned bove;

7 By that-is nieant that the anode i of the electron discl'iarg'er valve 42 is connected to the cathode 45 of the electron discharge valve 43. Similarly, the anode -44 of the electron discharge valv'e'43 is connectedto the cathode 45p! the electron discharge valve :42.- When these valves are ren-' deredfully conducting a path is provided for bothhalf cycles of the alternating current from the source 25 'so that the alternating current power reaches the-primary winding 38-01? the rectifier transformer 29 with little or no voltage drop and the rectifier'unit 28 will have a maximum direct current :output with the result that the oscillation generator-I i will operate: at maxi-' mum power output; In other words, under these conditions the; control arrangement functions as if the electron discharge unit 21 were omitted from the circuit. It is apparent that by'means of a suitable control potential applied to the grids or control-electrodes of the electron discharge valves 42 and 43, the instant of firing durin h alternating current cycle of these electron dis-.

charge valves. can be controlled as desired. By advancing the phase of the grid potentials applied to the control electrodes 46 relative to the anode or plate voltages applied to the anodes 44, theelectron discharge valves 42 and 43 canbe fired or rendered conducting early in the cycle with the. result that a greater plate potential. is available at the input terminals of the oscillation generator 11.; than in the case where the phase of the grid potentials applied to the control. electrodes 46 is. retarded. Consequently; oscillations of greater amplitude will result or in other words, the high frequency current flowing in the inductor orheating. coil 10 will have a greater amplitude. Conversely by retarding the. phase of the gridvoltage of the electron discharge valves 42 and 43 the magnitude of the current flowing in the inductor I is decreased.

In order to control the phase of thegrid potentials applied to the control electrodes 48 of the electron discharge valves 42 and 43, these control electrodes are provided with a suitable control circuit as follows. "The control electrode 46 of p theelectron discharge valve 42 is connected to its associated cathode 45 through a current limiting resistor 41, a bias battery 48 and the secondary winding 49 of 'a suitable. grid transformer designated at 50. Similarly, the control electrode 46 of the electron discharge valve 43 isconnected to its associated cathode 45 through.- ;a current limiting resistor 51in bias. 'b' attery 52 and a secondary winding 53 "and forming a part of. the

grid transformer 50 which'is providedfwithf a; primarywinding 54 inductively coupled to the secondary windings 49 {and 53. When the primary Winding 54 energized with an enema ingcurrent' potential then there are thes'econdary windingsfilfl and 53 alternating currenti'potentials which are displaced from each other by 1'80 electrical degrees. It is duite obvione that the electrondischarge Ve di/"e12 be. rendered conducting" during one-half cycle of the alternatingcin'rent ffoinlpotential source While. the'electron discharge valve43f will be rendered conducting during the otherghal'jg.- cycle of the alternating current potential. Whenever oneof these 'electrondischarge valves 42, or 43 charge valves 2' "and u being {rendered conducting unleks' an additional grid voltage s superimposed on the control electrodes iron; the grid transformer 50. By varying the phase of the aIter'nating current potential appliedfto the primary '-wine1ng'-s4 or the grid transformer relative "to the "potential of 'the alternating current-source 25, the conductivity of the electron dischar'g'e valves 42 and- 43 maybe controlled in any desired manner.

For' the purpose of supplyingprimary "winding- 54 of thegrid transformer 50 with analternating potential whichmay be varied in phase relative to the alternating potential of the source 25,- there is provided a phase shifting unit or device generally indicated at 60. It should-be understood that the construction of the phase shifting devicei0- iorzns nopart or the present invention and' may comprise any conventional phase shiftingdevice: To illustrate the invention, lhoweven. thephase shifting-device is shown as crime Etype disclosed-in Alexanderson Patent No.

1,719,866 including awinding 8| connected across potential source25 through a suitable adjustable resistor 62. The winding 6! is provided with end taps 83 and 64 and a mid-tap 65. 'The portion of the winding'il between tabs 6'3 and desig" nated as iF-l a, while the "portion betweenthe taps Strand 54 is designated as Blb. A bridge circuit is defined with two of the legs comprising wind ing' poi'tions-fila 'and 6lbwhi1e the other'ftwo legs comprise a resistor '66 ana -a variable inducinductance G1 is connected between the'terminalsj s4 and? The terminals-s5 amiss provide the outputwterminalsdor the bridge-circuit of'the' phaseshifting device it and are connected across the primary winding 5401' the grid mummies -50;;through a suitable-current limiting resistor suitable source ofwpotential such as the battery" Hithrougha suitable-variable or control resistor 14-, It will be understood that when the resistor Hiswaried so that a large saturating current flows in the'saturatingwinding 12 that them ductance of the :legof the bridge circuit between the terminals :64 and-58 is decreased. On the other hand, when the current flowing in the saturating winding 12 is decreased by varying the resistance of the resistor the inductance of this leg of the bridge circuit isincreased.

The operation of the phase shifting unit or tie--- vice fifl iwill be: apparent from an examinaticmof the vectordiagramslshownzinFigs. '4 and'5.:of..the

drawings. lhese vector diagrams represent the voltage conditions-existing across'the legs of the bridge circuit of the phase shifting device at two diiierent values of the current flowing in the saturation winding 12 of the saturable reactor 61. Each .vector is designated by the letter V in Fig.1

ref the drawings marked with an appropriate subscript corresponding to the particular leg of the bridge circuit of Fig. 3 of the-drawings. The

samenomenclature is need in Fig. 5 of the draw I ings except that the voltage vectors other than v61 are designated with a prime in-order. t on tinguish them from the corresponding vectors in Fig. 4 of the drawings. Reierring specifically to Fig. 4 of the drawings, the vector .Veirepresents the instantaneous voltage of the. source :25 as applied across the terminals .53 and 64 of the winding 6|. It will be apparent that half of this voltage will appear across the section Bla of the winding and the other half will appear across the section Blb of the winding. The voltage vectors Ves and V67 which are displaced from each other by 90 electrical degrees represent the voltages across the resistor 68 andthe saturable reactor 61 respectively for one value of current flowing through the saturating winding 12. The vector V54 is the output voltage obtained across the primary winding 54 of the grid transformer ill -for the particular current flowing through the saturating winding 12 of the saturable reactorfifl. The corresponding vectors marked with aprime in Fig. 5 of the drawings represent the; same voltages for a difierent current condition withrespect to the saturating winding 12. It is appar--% ent that by varying the current flowing inthe saturating winding 12 and consequently by varying the inductance of the leg of the bridge-circuit between the terminals 64 and 68 the voltage vectors Vee and V6! vary in magnitude and phase. They are, however, always displaced from each other by 90 electrical degrees and the junction point of these two vectors always remains on the dotted curve G which is a semicircle, With this arrangement, the phase of'the voltage rep-* resented by the vector V54 in Fig. 4 of the draw-' ings and V54 in Fig. 5 of the'drawings mayvary" widely through an angle of substantially 180 ide-- grees from a value in phase with the voltage of the source 25 represented bythe vector V61 which is the most advanced phase position to moreand more retarded phase positions relative to the volt-;:

age V61. Since a high saturating-current in the winding 12 causes a decrease in inductance of the: alternating current windings 10 and H of the saturable reactor 61, a lowxvoltage will appear across this leg of the bridge circuit. This condi: tion is represented by'Fig. 5 of 'the drawings which indicates that an increase in the saturat-" ing current flowing in the winding 12 causes an advance in the phase of the voltage across the winding 54 since the vector Vsi inFig- 5- is-substantially in phase with the vector V61 or at least much more nearly in phase than isthe vector V64 of Fig. 4 of the drawings relative to-this same source voltage Val; Fig. 4 represents the condition when the saturating current flowing in the winding 12 is relatively low whereby the saturable reactor presents a high inductance between the terminals 84 and 6B of the bridge network. i a

From the above discussion, it will be apparent that by varyingthe resistance of resistor 14' there is produced a variation of'the'firing tangle of the electron'discharge valves 42 and 43soas I 0 is caused to transmit maximum power ,at. a

below the desired end temperature thereof wherebyvthe total heatingtimeto reach the desiredendtemperatureis a minimum without ever;ex-.

ceeding the maximum heating rate. In View of the detailed description included above, the operation of the control arrangement for the high frequency heating device-will be obvious to those skilled in the art andno further discussion there of willbe included herewith. It should be understood that similar results can'be' obtained by varying the resistance-of the resistor 66 and permitting the inductance ofthe reactor 61 to remain constant,

Instead of controlling the current flowing in the inductor coil Ill by manually varyingthe resistance of the resistor 14 or by controlling the resistance of thelresistor 'lll throug'h asuitable program timer of some sort, automatic control responsive to some characteristic "or f-unction' of the oscillationgenerator ll maybe employed. Such an arrangement is shown in Fig.6 of :the; drawings whereonlya portion of the control oircult of Fig. 3- is'illustrated. However, since a large portion of it lei-identical with that of Fig.3 of the drawings it has been omitted and as far as the parts shown 'in-; Fig. .6 which correspondto the parts shown 'in Fig.3 are concerned, the same reference numerals are applied. The grid" leak resistorEZ is illustrated as being comprised of two parts 22a, and;-22b-grounded at ll as clearly indicated in Fig; .6 ofv the drawings there-'- by being effectively .oonnected'to the cathode-l4 of the electron discharge valve l2 which is" also grounded at 23. .In order to' obtain the directcurrent for the saturatingwinding T2, the re sistor 22b is provided with a variable tap 18 which is connected to'one terminal of the satu-'-' rating winding 12a:- The otherterminal of the saturating windin -J2 is connected to oneend of the resistor 72% such as the'ground point 11 A suitable-capacitor- 19 ;is; provided connected across the'saturating winding lilto by-pass high" frequency components sotthat the saturating winding '12 issuppliedxsubstantially with-adi rect current potential;-: ,It will be understood that'if .the load on theosoillation generator ll"- increases oris tooxhigh, the grid current of'the electron discharge valve l2 decreases. "Conse-" quently, if the load is high the gridcurrent-is low and the currentfiowing through the saturat ing winding TZ'taReu fromlresistor 22b is low thereby increasing the impedance of saturabl'e" reactor er, thus retarding thepha's'e or the tential appliedito the-grid transformer n relative .to the potential'of the source 25 and conse quently decreasing the output. 'Conversel'yif the degree of loading of the oscillation generator; I l "decreases, the "grid" current increases, thereby increasing the saturation of the saturable reactor 61' by increasing the current flowing through the windinglr and decreasing the iii' ductance of 'th'elegof the-' bridge circuit between theterminals 6'4"a'nd68. This: decrease in ma"- ductan'ce as is'jappa rent' from Figs. 4 and 5 the drawing's advances'the phaseof the voltage" applied to the 'contrb'l electro'des id'of'the leietron'dis'char'ge'valves' 562' and t3 relative to the? platevolt'ages'thereof so as to increasethe load on the oscillation generator H, thereby auto-f matica1ly-'providingfthe desired regulation.

It may be undesirableto draw power from the grid circuit of the electron discharge valve 12 tocontrol the saturationpf the saturable wind ing 12.- Aecordingly, in Fig. 7 there;is disclosed} anarrangernent substantially identical with that disclosed Fig. of the drawingswhich elim i hates the requirementeofldrawing power from the grid circuit of the electron discharge valve including a vacuum tube for producing high frequency oscillations in said heating coil and a source of alternating current potential, means including an electronic control unit and rectifier interposed between said source and said oscillation generator so that a variable direct current.

potential is supplied to said oscillation generator, said electronic control unit comprising a pair of three element electron discharge valves having their plate current circuits connected to said source, means for controlling the instant during the cycle of the alternating current potential of said source that said electron discharge valves are rendered conductive and means responsive to a function of the grid current of said vacuum tube for controlling said last-mentioned means to tend to maintain the power supplied to said heating coil constant during a heating cycle.

4. For use in a high frequency heating device comprising a heating coil and an oscillation generator including a vacuum tube and tank circuit connected to supply said heating coil with high frequency oscillations and a source of alternating current potential, means including an electronic control unit and rectifier interposed between said source and said oscillation generator so that a variable direct current potential is supplied to said oscillation generator, said electronic control unit comprising a pair of three element electron discharge valves having their plate current circuits connected to said source, phase shifting means for controlling the instant during the cycle of the alternating current potential of said source that said electron discharge valves are rendered conductive, and means responsive to a current characteristic of said vacuum tube for controlling said last-mentioned means to tend to maintain the power supplied to said heating coil constant during a heating cycle.

5. In a control arrangement for controlling the current supplied between two circuits comprising, a load circuit, an alternating current supply circuit, an oscillation generator interconnecting said circuits, a pair of electron discharge valves arranged in back-to-back connection in series with one Of said circuits, a control circuit for said electron discharge valves including an impedance bridge type phase shifting unit connected to said supply circuit, one element of said phase shifting unit having a variable impedance whereby the conductivity of said electron discharge valves may be varied, and means for automatically causing the impedance of said element to vary in response to a predetermined current characteristic of said oscillation generator.

6. In a control arrangement for controlling the power supplied between two circuits comprising, a load circuit, an alternating current supply circuit, an oscillation generator interconnecting said load and supply circuits, a pair of electron discharge valves arranged in back-to-back connection in series with one of said circuits, a control circuit for said electron discharge valves including an impedance bridge type phase shifting unit connected to said supply circuit, one element of said impedance bridge type phase shifting unit comprising a saturable reactor, a saturating circuit for said reactor to vary the impedance thereof, and means for controlling said saturating circuit in a predetermined manner to tend to maintain constant the power supplied to said load circuit.

7. For use in a high frequency heating device comprising a load circuit including an oscillation generator for generating high frequency oscillations and a high frequency heating coil connected to said generator and arranged to be inductively coupled with a work piece having a positive temperature coefiicient of electrical resistivity and a source of alternating current potential connected to said load circuit through an electric circuit breaker, a pair of electron discharge valves arranged in back-to-back connection in series with said load circuit, and a control circuit for said electron discharge valves including an impedance bridge type phase shifting unit connected to said source so that the conductivity of said electron discharge valves may be varied by varying the impedance of one element of said phase shifting unit, and means for varying the impedance of said one element in response to a current characteristic of said oscillation generator so that the power supplied to said load circuit remains substantially constant throughout a heating cycle regardless of the change in temperature of said work piece.

8. For use in a high frequency induction heating device comprising a, heating coil and an oscillation generator including a vacuum tube connected to supply said heating coil with high frequency oscillations and a supply circuit comprising a source of alternating current potential, a rectifier unit interconnecting said source and said generator for transforming said alternating current potential to direct current potential, said rectifier unit, oscillation generator and heating coil comprising a load circuit, a circuit breaker connected between, said load and supply circuits, a pair of electron discharge valves connected in back-to-back relationship and in series with said load circuit, a control circuit for said electron discharge valves including an impedance bridge type phase shifting unit connected to said supply circuit, one element of said impedance bridge type phase shifting unit comprising a saturable reactor, a saturating circuit for said reactor to vary the impedance thereof and means for controlling said saturating circuit in response to a function of the grid current circuit of said vacuum tube.

HERBERT F. STORM.

REFERENCES CITED lhe following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,948,704 Fischer Feb. 2'7, 1934 2,057,485 I-Ialler Oct. 13, 1936 2,147,689 Chafiee Feb. 21, 1939 2,151,753 Etzrodt Mar. 28, 1939 2,175,694 Jones, Jr Oct. 10, 1939 2,231,457 Stephen Feb. 11, 1941 2,251,277 Hart et al. Aug. 5, 1941 2,293,851 Rogers Aug. 25, 1942 2,324,525 Mittelmann July 20, 1943 2,391,085 Crandell Dec. 18, 1945 2,400,472 Strickland May 14, 1946 2,415,799 Reifel et al Feb. 11, 1947 2,416,172 Gregory et al Feb. 18, 1947 2,441,435 Mittelmann May 11, 1948 FOREIGN PATENTS Number Country Date 439,166 Great Britain Dec. 2', 1935 OTHER REFERENCES Electronics, February, 1945, vol. 18, No. 2, pages -115. 

